VI.] WATER REQUIRED BY CROPS 107 



evaporation at the surface, but yet are sufficiently coarse 

 to admit of the reasonably free movements of water 

 either downwards or upwards. There is, in fact, a happy 

 medium of grain which has a large and effective surface 

 that is yet not too large to shut down the movement of 

 the water. Such soils are found among some of the 

 loams and fine sands in which predominate those 

 fractions that we have previously designated as fine 

 sand and silts. 



We have already discussed the amount of water 

 required by the growing plant, and have found that 

 taking the experimental figure of 200 to 300 pounds 

 of water transpired from each pound of dry matter 

 elaborated, then the usual crops grown in this country 

 will evaporate from 6 to 10 inches of rain, and even 

 more in the case of a heavy crop of mangolds. Con- 

 sidering that this growth is all made during a portion of 

 the year only, that evaporation from the bare soil will 

 also dissipate much of the rainfall, a great deal of which 

 also runs off the surface or percolates so quickly into 

 the subsoil as to lead to its permanent loss to the land 

 by drainage, we can easily understand that where the 

 annual rainfall is only 30 inches or so, as is the case 

 over the greater part of England, the crops are often 

 likely to suffer for want of water. Humid as is the 

 English climate, the magnitude of the crops in any given 

 year is more often determined by lack of water than by 

 any other single factor, and the majority of the 

 operations of cultivation are directed towards obtaining 

 and storing as large a water-supply as possible. 

 Beginning in the autumn, it is desirable to get the 

 stubbles ploughed early and left rough for the winter ; 

 much more of the winter rainfall will be absorbed by 

 such a ploughed surface than by one which remains so 

 firm and trampled by the harvest operations that water 



